1. Field of the Invention
The present invention relates to a broadband wireless communications system. More particularly, the present invention relates to an apparatus and method for performing channel estimation using pilots and a preamble in a broadband wireless communications system supporting Space Division Multiple Access (SDMA).
2. Description of the Related Art
With the dawn of the wireless multimedia era, demand for high-speed wireless transmission of large amounts of data has rapidly increased. This increasing demand has become a driving force behind worldwide active research concerning high-speed wireless data transmission systems to provide Internet service using radio channels.
While 3rd Generation (3G) communications systems support up to 2 Mbps for stationary users, 4th Generation (4G) communication systems aim to provide 1 Gbps for stationary users or pedestrians within a Wireless Local Area Network (WLAN) environment and 100 Mbps for vehicles within a Wireless Metropolitan Area Network (WMAN) environment. However, since wireless high-speed data transmission using radio channels surfers from a high error rate due to multipath interference, a radio access technique suitable for radio channels is needed.
In order to reduce errors caused by the multipath interference of radio channels, Institute of Electrical and Electronics Engineers (IEEE) 802.16d systems have been developed in which the physical channels of the WMAN system operate in Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA). IEEE 802.16d systems are defined in the IEEE 802.16d wireless communications standard, the entire disclosure of which is hereby incorporated by reference. These IEEE 802.16d systems realize high-speed data transmission by sending physical channel signals on a plurality of subcarriers. However, IEEE 802.16d systems are only for fixed wireless connections.
The IEEE 802.16e wireless communications standard, the entire disclosure of which is hereby incorporated by reference, is an amended variant of IEEE 802.16d and is expected to enable connections for mobile devices. IEEE 802.16e offers a standard mechanism to support an Adaptive Antenna System (AAS) mode and a non-AAS mode in one frame. The benefit of applying the AAA mode to systems is an increase in both cell capacity and cell coverage.
The AAS adaptively forms a beam pattern in accordance with a varying radio channel environment by continuously monitoring a cell area. Given one Mobile Station (MS) and no interference, the AAS creates an effective antenna pattern that tracks the movement of the MS, thereby adapting to the movement of the MS. The antenna pattern has the highest gain in the direction of the MS. With this AAS, SDMA can be implemented. As such, given N MSs, N beams of the same frequency band are steered in the directions of the individual MSs. Thereby, the MSs share the same time and frequency resources.
FIG. 1 illustrates a conventional SDMA cellular system. Referring to FIG. 1, a Base Station (BS) 101 in a smart antenna system forms different beams 102 and 103 for MSs at different locations. The beams 102 and 103 are formed in accordance with corresponding beam patterns in order to spatially divide the same frequency and time resources. To form the spatially divided downlink beams, the BS 101 needs uplink channel information. Thus, AAS preamble symbols are added on the downlink and the uplink to support the AAS mode in the IEEE 802.16e OFDMA system, as illustrated in FIG. 2.
FIG. 2 illustrates the structures of a downlink frame 201 and an uplink frame 202 in the conventional IEEE 802.16e OFDMA system.
Referring to FIG. 2, an AAS preamble is delivered at the start of downlink and uplink bursts. For downlink beamforming, the BS 101 performs channel estimation using the uplink AAS preamble and estimates a beam coefficient based on the estimated channel information. The BS 101 then forms a beam using the beam coefficient. Since spatial channels 102 and 103 resulting from the beamforming are divided in space, as shown in FIG. 1, they do not interfere with one another despite sharing the same frequency and time resources. Consequently, each spatial channel fully utilizes the frequency and time resources assigned by the system.
The uplink preamble is used in compliance with the physical layer standard of the IEEE 802.16e OFDMA system as follows.
According to the physical layer standard of the OFDMA system, AAS preambles assigned to SDMA users utilize the same Pseudo Noise (PN) code. In OFDM modulation of the PN code sequence, therefore, each SDMA user multiplies a specific user signature by the PN code sequence, for user identification. The user signature is given as
                    a        =                              1                          N                                                tile                  ⁡                                      (                    bin                    )                                                  ⁢                                  _                  ⁢                  size                                                              ×                      [                                                                                ⅇ                                          j                      ⁢                                                                        2                          ⁢                          π                          ⁢                                                                                                          ⁢                                                      f                            ⁡                                                          (                              k                              )                                                                                ⁢                          m                                                                          N                                                                                    tile                              ⁡                                                              (                                bin                                )                                                                                      ⁢                                                          _                              ⁢                              size                                                                                                                                                                                                                                            ⅇ                                          j                      ⁢                                                                        2                          ⁢                                                      π                            ⁡                                                          [                                                                                                f                                  ⁡                                                                      (                                    k                                    )                                                                                                  +                                1                                                            ]                                                                                ⁢                          m                                                                          N                                                                                    tile                              ⁡                                                              (                                bin                                )                                                                                      ⁢                                                          _                              ⁢                              size                                                                                                                                                                                                                        ⋮                                                                                                  ⅇ                                          j                      ⁢                                                                        2                          ⁢                                                      π                            ⁡                                                          [                                                                                                f                                  ⁡                                                                      (                                    k                                    )                                                                                                  +                                                                  N                                                                                                            tile                                      ⁡                                                                              (                                        bin                                        )                                                                                                              ⁢                                                                          _                                      ⁢                                      size                                                                                                                                      -                                1                                                            ]                                                                                ⁢                          m                                                                          N                                                                                    tile                              ⁡                                                              (                                bin                                )                                                                                      ⁢                                                          _                              ⁢                              size                                                                                                                                                                                                ]                                              (        1        )            where Ntile(bin)—size denotes the number of tones per tile (or bin) (9 tones for an Adaptive Modulation and Coding (AMC) bin and 4 tones for a Partial Usage SubCarrier (PUSC) tile) and f(k) denotes the first tone index of a kth tile (or bin) in the AAS preamble. m is a user index, representing cyclical time shifts for the SDMA user (e.g. 0, 1, 2, 3, . . . , Ntile(bin)—size−1). That is,
  ⅇ      j    ⁢                  2        ⁢        π        ⁢                                  ⁢                  f          ⁡                      (            k            )                          ⁢        m                    N                              tile            ⁡                          (              bin              )                                ⁢                      _            ⁢            size                              denotes the phase rotation of a tone with the tone index f(k), determined by m.
The user signature expressed in Eq. (1) leads to a linear phase rotation of the PN code sequence for the user in the frequency domain. This is equivalent to cyclical time shifting at a predetermined interval in the time domain. For a PUSC or AMC channel with clustered tones, the user signature functions to assign an orthonormal kernel between SDMA users.
FIG. 3 is a block diagram of an uplink channel estimation apparatus in a BS in a conventional SDMA-OFDMA system. Because the channel estimation is performed in the same manner with respect to each antenna, the channel estimation for one antenna is shown and described by way of example.
Referring to FIG. 3, the BS downconverts a Radio Frequency (RF) signal received through an antenna 301 to a baseband signal and converts the baseband analog signal to a baseband digital signal.
A Cyclic Prefix (CP) remover 303 removes a CP from the baseband digital signal. A Fast Fourier Transform (FFT) processor 305 converts the time-domain signal received from the CP remover 303 into a frequency-domain signal by using a N-point FFT.
An AAS preamble tone extractor 307 extracts an AAS preamble signal from the frequency-domain signal that corresponds to an area assigned to the SDMA user. A preamble code multiplier 309 removes a PN code multiplied by the tones of the preamble signal by
                              y          –                =                              [                                                                                                      y                      –                                        1                                                                                                                                          y                      –                                        2                                                                                                ⋮                                                                                                                        y                      –                                        n                                                                        ]                    =                      c            ·            y                                              (        2        )            where y denotes the FFT signal from the FFT processor 305, c denotes a PN code vector for the AAS preamble, and yn denotes an nth AMC bin (or PUSC tile) multiplied by the PN code vector. Here,
            y      _        n    =                    [                                            y              _                        ⁡                          (                              f                k                            )                                ,                                    y              _                        ⁡                          (                              f                                  k                  +                  1                                            )                                ,          …          ⁢                                          ,                                    y              _                        ⁡                          (                              f                                  k                  -                  1                  +                                      N                                                                  tile                        ⁡                                                  (                          bin                          )                                                                    ⁢                                              _                        ⁢                        size                                                                                                        )                                      ]            T        .  That is, only the phase rotation value of the SDMA user remains by multiplying the FFT signal by the PN code vector of the AAS preamble.
The number of channel estimators 311 to 313 provided is the same as the number of AMC bins (or PUSC tiles). The channel estimators 311 to 313 calculate average channel estimates representative of their corresponding AMC bins (or PUSC tiles) by computing Eq. (3) using the output of the preamble code multiplier 309.
Channel estimation is carried out on a bin-by-bin basis (or on a tile-by-tile basis) byH(fn)=aH yn  (3)where H(fn) denotes a channel estimate representative of an nth bin (or tile), yn denotes the nth AMC bin (or PUSC tile) multiplied by the PN code vector according to Eq. (2), a denotes a unique orthonormal kernel between SDMA users described in Eq. (1), and aH denotes the Hermitian of a.
As described above, the general frequency-domain channel estimation is performed by cross-relation of orthonormal kernels between SDMA users as expressed in Eq. (1), Eq. (2) and Eq. (3) in the SDMA-OFDMA system. Thus, the channel estimate of each bin (or tile) is achieved. This channel estimation information is used when calculating a beam coefficient for the data area.
Meanwhile, if the channel conditions varies greatly over time, application of the channel information estimated by the preamble to data in a different time area degrades performance. For example, in the IEEE 802.16e OFDMA system which takes into account mobility, the variation of the channel condition increases with the mobility of the MS. Therefore, the performance degradation caused by application of the channel estimation estimated by the preamble to beamforming for a data area that is different in time, reduces the gain achieved by using the smart antenna system.
Accordingly, there is a need for an improved apparatus and method with improved channel estimation performance that increases the gain of a smart antenna system.